Generally, in an organic light emitting device known and described as an organic electroluminescence device (an organic EL device), an anode which is a transparent electrode, a hole transport layer, an organic emission layer, an electron injection layer, and a cathode are stacked on one side of a transparent substrate in this order. When a voltage is applied between the anode and the cathode, an electron injected into the emission layer through the electron injection layer and a hole injected into the emission layer through the hole transport layer recombine in the emission layer, and an excited state occurs and light is emitted. The light emitted from the emission layer is taken out through the transparent electrode and the transparent substrate.
In recent years, in such an organic light emitting device, a so-called multiphoton device in which a plurality of organic emission layers are stacked between the anode and the cathode and an equipotential surface forming layer (an equipotential surface forming layer) or a charge generating layer is provided between each adjacent emission layers has been proposed to achieve high intensity emission and a long life span (see Japanese Non-examined Patent Publication No. 11-329748, Japanese Non-examined Patent Publication No. 2003-45676, Japanese Non-examined Patent Publication No. 2003-272860, and so on).
FIG. 12 shows one example of a structure of an organic light emitting device formed as such a multiphoton device, in which a plurality of emission layers 3 are stacked between an anode 1 and a cathode 2 in a condition where an equipotential surface forming layer 4 or a charge generating layer 4 is provided between each adjacent emission layers 3, and they are stacked on a surface of a transparent substrate 10. The anode 1 is formed as an optically-transparent electrode and the cathode 2 is formed as a light reflective electrode. Although a hole transport layer and an electron injection layer are formed on both sides of the emission layer 3, the hole transport layer and the electron injection layer are not shown in FIG. 12. By separating the plurality of emission layers 3 by the equipotential surface forming layer 4 or the charge generating layer 4, the plurality of emission layers 3 can emit at the same time as if they are connected in series, and the light from each emission layer 3 are combined, whereby high current efficiency and quantum efficiency which were impossible for a conventional organic light emitting device (organic EL device) can be realized, and high intensity emission can be achieved (see Japanese Non-examined Patent Publication No. 2003-45676, and Japanese Non-examined Patent Publication No. 2003-272860).
The feature of the organic light emitting device structure resides in that; it is a thin film device having a film thickness on the order of optical wavelengths; it has a refractive index step or a reflecting surface formed by a metal surface inside the device; and it emits light from an emission layer which is a high refractive index medium. In this structure, a phenomenon of an optical interference effect or light confinement in a high refractive index medium, such as the emission layer of an organic film, the substrate, and the electrode, by a total reflection may be occurred, and as a result, angle dependency of emission brightness and emission spectrum, film thickness dependency, and deterioration of light use efficiency are observed. This problem is occurred in the above organic light emitting device which is the multiphoton device having a plurality of emission layers. As to the optical interference effect, it becomes possible to realize an improvement of chromatic purity and to realize control of directional characteristics, and so on, by using it appropriately, and particularly, it is useful for use in a flat panel display and the like. For example, Japanese Non-examined Patent Publication No. 7-240277 and Japanese Non-examined Patent Publication No. 2000-323277 disclose a fact that it is possible to emphasize a wavelength by adjusting an optical distance between the emission layer and the light reflective electrode to an even number multiple of the ¼ wavelength or by adjusting an optical distance between the emission layer and a maximum refractive index step position to an even number multiple of the ¼ wavelength, and particularly, it is known that the optical distance between the emission layer and the light reflective electrode have a great influence on the emission spectrum. The above Japanese Non-examined Patent Publication No. 2003-272860 further discloses a fact that it is possible to obtain light emission of highest efficiency and a fact that a shape of the emission spectrum gets thin by setting all optical film thicknesses between an emission point of each of the emission layers and the light reflective electrode to odd numbers multiple of the ¼ wavelength.
However, in the organic light emitting device in which the chromatic purity and so on were improved by optimizing the optical distance between the emission layer and the light reflective electrode or the optical distance between the emission layer and the maximum refractive index step position, namely, the film thickness of the device, fluctuations of the emission brightness and an emission color become large when the film thickness varies. This means that an allowable fluctuation of the film thickness in a manufacturing process of the organic light emitting device becomes smaller, and it is directly linked to a productivity problem. Particularly, in the above organic light emitting device having a structure in which the emission layers and the equipotential surface forming layers or the charge generating layers are stacked, accuracy and inevitability of the film thickness control further increase because film thickness abnormality of any layer may affect on even the optical position of other layers.
Furthermore, in the above Japanese Non-examined Patent Publication No. 2003-272860, although the optical distance between the emission layer and the light reflective electrode was set to an even number (2n+1, where n=0, 1, 2, . . . ) multiple of the ¼ wavelength, it is known that angle dependency of the brightness and the spectrum increases with the increase of the value of “n”. That is, in a organic light emitting device having only one emission layer, the fluctuation of the emission brightness and the emission color with respect to the film thickness variation is not necessarily large because such a device is often designed in the optical distance equivalent to “n=0”, but in the above organic light emitting device having multiple emission layers, because each emission layer is located at a position of (2n+1) times the ¼ wavelength, a particular wavelength is remarkably emphasized with the increase of the number of layers, and as a result, there is a problem that an emission spectrum that is greatly different from a spectrum that the emission layer originally has may be given and the angle dependency becomes large.
Therefore, although the above organic light emitting device having multiple emission layers separated by the equipotential surface forming layer or the charge generating layer can certainly realize high current efficiency and quantum efficiency that were impossible for a conventional organic light emitting device, it does not necessarily have a desirable characteristic with respect to the emission spectrum and the angle dependency.
In order to solve such problems, it is described in the above Japanese Non-examined Patent Publication No. 2003-272860 that, in the organic light emitting device having a plurality of emission layers, the optical interference effect is cancelled by absorbing a part of generated light by a light absorption means or by diffusely reflecting it by a light diffuse reflection means, and therefore, adjustment of the optical film thickness between the emission point and the light reflective electrode becomes practically unnecessary. However, in the above Japanese Non-examined Patent Publication No. 2003-272860, it does not refer to the problem of the angle dependency which the organic light emitting device has, and the problem has not been solved yet.